Nanoscale scanning sensors

ABSTRACT

A sensing probe may be formed of a diamond material comprising one or more spin defects that are configured to emit fluorescent light and are located no more than 50 nm from a sensing surface of the sensing probe. The sensing probe may include an optical outcoupling structure formed by the diamond material and configured to optically guide the fluorescent light toward an output end of the optical outcoupling structure. An optical detector may detect the fluorescent light that is emitted from the spin defects and that exits through the output end of the optical outcoupling structure after being optically guided therethrough. A mounting system may hold the sensing probe and control a distance between the sensing surface of the sensing probe and a surface of a sample while permitting relative motion between the sensing surface and the sample surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/675,156, filed on Feb. 18, 2022, which is a continuation of U.S.application Ser. No. 15/965,175, filed on Apr. 27, 2018, which is acontinuation of U.S. application Ser. No. 14/423,123, now U.S. Pat. No.10,041,971, filed on Feb. 21, 2015, which is a 35 U.S.C. § 371 NationalPhase Entry Application of International Application No. PCT/US13/55644,filed on Aug. 20, 2013. International Application No. PCT/US13/55644designates the U.S. and claims the benefit of priority under 35 U.S.C. §119 from U.S. Provisional Application Ser. No. 61/692,077, filed on Aug.22, 2012, entitled “Nanoscale Scanning Sensors.” The contents of each ofthese applications are incorporated herein by reference in theirentireties as though fully set forth.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under contract number60ANB10D002 awarded by the NIST; contract number HR001 1-09-1-0005 byDARPA; and contract number HR0011-10-1-0073 by DARPA. The government hascertain rights in the invention.

BACKGROUND

Spin defects in solid state systems, such as the NV (nitrogen-vacancy)defect centre in diamond, have numerous potential applications. Theseapplications include without limitation, nanoscale electric and magneticfield sensing, single photon microscopy, quantum information processing,and bioimaging.

NV-centre based nanosensors rely on the ability to position a singlenitrogen-vacancy centre within a few nanometers of a sample, and thenscan it across the sample surface, while preserving the NV centre's spincoherence and readout fidelity.

Existing scanning techniques, however suffer from drawbacks that includelow sensitivity low resolution and high data acquisition times. It isconsidered that these drawbacks are due to a number of factors includingone or more of: short spin coherence times due to poor crystal quality;too large a distance between the spin defect and the sample surfacebeing analyzed; variations in the distance between the spin defect andthe sample surface being analyzed; and inefficient far-field collectionof the fluorescence from the NV centre.

For example, one known technique utilizes a diamond nano-particlecontaining an NV spin defect. The diamond nanoparticle is adhered to anoptical fiber to optically address the NV defect within the diamondnano-particle, a microwave generator is utilized to manipulate the spinstate of the NV defect when the diamond nano-particle is placed in closeproximity to a sample to be analyzed, and a detector is provided on anopposite side of the sample to detect fluorescence from the NV defect.

The aforementioned configuration has a number of problems. First, whilethe use of a diamond nano-particle ensures that the NV defect can bepositioned close to the sample to be analyzed, diamond nano-particlestend to be of poor diamond quality and the NV defects therein have shortspin coherence times and can be optically unstable leading to poorsensitivity. Secondly fluorescent light is emitted in all directions andonly a small proportion can be detected. Thirdly, the detector isdisposed on an opposite side of the sample to the diamond nano-particleand thus the configuration can only be used for material samples whichare transparent to the fluorescent emission. While the optical detectorcould be positioned on the same side of the sample as the diamondnano-particle, it is difficult to arrange the detector to effectivelycapture fluorescent emission because the diamond nano-particle isadhered to the end of an optical fiber which inhibits detection offluorescence on the same side of the nano-particle as the optical fiber.

An alternative to the aforementioned configuration would be to use ahigh quality single crystal diamond material comprising an NV defectwhich has a longer spin coherence time. However, the use of a micronscale single crystal diamond material has a number of problemsincluding, for example: too large a distance between the spin defect andthe sample surface being analyzed; variations in the distance betweenthe spin defect and the sample surface being analyzed; and inefficientfar-field collection of the fluorescence from the NV centre.

It is an aim of certain embodiments of the present invention to solveone or more of the aforementioned problems.

SUMMARY OF INVENTION

According to one aspect of the present invention there is provided asystem comprising: a sensing probe formed of a diamond materialcomprising one or more spin defects configured to emit fluorescentlight, said one or more spin defects being located no more than 50 nm ofa sensing surface of the sensing probe, the sensing probe furthercomprising an optical outcoupling structure formed by the diamondmaterial, the optical outcoupling structure configured to opticallyguide the fluorescent light emitted by the one or more spin defectstoward an output end of the optical outcoupling structure; an opticalexcitation source configured to generate excitation light directed tothe one or more spin defects causing the one or more spin defects tofluoresce; an optical detector configured to detect the fluorescentlight that is emitted from the one or more spin defect and that exitsthrough the output end of the optical outcoupling structure after beingoptically guided therethrough; and a mounting system configured to holdthe sensing probe so as to control a distance between the sensingsurface of the sensing probe and a surface of a sample while permittingrelative motion between the sensing surface of the sensing probe and thesample surface.

According to a second aspect of the present invention there is provideda sensing probe for use in the aforementioned system. The sensing probeis formed of a diamond material and comprises: one or more spin defectsconfigured to emit fluorescent light; and an optical outcouplingstructure formed by the diamond material, the optical outcouplingstructure configured to optically guide the fluorescent light emitted bythe one or more spin defects toward an output end of the opticaloutcoupling structure, wherein the one or more spin defects are locatedno more than 50 nm from a sensing surface of the sensing probe.

Certain further aspects of the present invention relate to sensingmethods as described and claimed herein.

Certain embodiments of the present invention have improved sensitivity,higher resolution, and lower data acquisition times when compared withprior art arrangements. These advantageous features are achieved throughthe provision of a combination of: a small and controlled distancebetween spin defects and a sample surface being analyzed by locating oneor more spin defects very close to a sensing surface of the diamondmaterial while retaining the spin coherence properties of the spindefects; and efficient far-field collection of spin defect fluorescencethrough the provision of an optical outcoupling structure coupled to theone or more spin defects located close to the sensing surface.

The one or more spin defects may be located no more than 40 nm, 30 nm,20 nm, 15 nm, 12 nm, or 10 nm from the sensing surface of the sensingprobe. Typically the sensitivity of the system will be increased bylocating the one or more spin defects closer to the sensing surface as afield to be sensed will decrease in intensity with increasing distancefrom a sample surface. By locating the one or more spin defects closerto the sensing surface of the sensing probe then the one or more spindefects can be positioned closer to a sample surface thereforeincreasing sensitivity. Furthermore, resolution can also be improved byenabling the one or more spin defects to be located closer to a samplesurface.

Further improvements in sensitivity can be achieved through theprovision of spin defects which have relatively long spin coherencetimes due to the use of good quality diamond material (preferably highquality single crystal diamond material). It is not straightforward toprovide such long spin coherence defects close to a sensing surface asthe spin coherence properties of spin defects are detrimentally affectedby surface interactions and/or the processing steps required to processback a surface to reduce the surface-spin defect distance. As describedin more detail later in this specification, the present inventors havedeveloped processing techniques to fabricate optical outcouplingstructures with one or more spin defects located therein close to asensing surface while at the same time retaining the spin coherenceproperties of the spin defects. As such, the decoherence time of the oneor more spin defects may be greater than 10 μsec 50 μsec, 100 μsec, 200μsec 300 μsec 500 μsec, or 700 μsec.

In order to provide a system which has high resolution, it isadvantageous to provide relatively few, and ideally one spin defectlocated close to the sensing surface and coupled to the opticaloutcoupling structure. For example, the sensing probe may comprise nomore than 50, 30, 10, 5, 3, 2, or 1 spin defects located close to thesensing surface and optically coupled to the optical outcouplingstructure (e.g. by locating the spin defects within the opticaloutcoupling structure near a sensing surface thereof). In the case thatonly one near surface spin defect (or relatively few) are provided toimprove resolution, it is advantageous that such a spin defect has along decoherence time as previously described to improve sensitivity.

Alternatively to the above, if very high resolution is not a requirementfor certain applications then a larger number of spin defects may beprovided. For example, the sensing probe may comprise a plurality ofspin defects (e.g. more than 50) in the form of a layer located no morethan 50 nm from the sensing surface and optically coupled to the opticaloutcoupling structure. In this case, each individual spin defect is notrequired to have such a high decoherence time to achieve goodsensitivity due to the large number of individual spin defects acting assensing elements. As such, sensitivity can be retained but at theexpense of lower resolution.

In certain embodiments the sensing probe including the opticaloutcoupling structure is formed of a diamond component having at leastone linear dimension greater than I μm in length. For example, thesensing probe including the optical outcoupling structure may be formedof a micron scale (or even millimeter scale) single crystal diamondmaterial. Such a sensing probe has three advantageous over diamondnano-particles: (i) an optical outcoupling structure can more readily befabricated into a larger piece of diamond material; a detector can bemore readily located relative to the one or more spin defects and othercomponents of the system such as the optical excitation source andmounting system whereby detection can be achieved with efficiency on thesame side of a sample as the sensing probe; using larger scale, highquality diamond material enables the fabrication of better quality spindefects in terms of coherence time and spectral stability.

A number of possible optical outcoupling structure could be fabricatedinto the diamond material of the sensing probe including a nanopillar ora solid immersion lens. Optical outcoupling can also be effected viainternal reflection, i.e. using the macroscopic shape of the diamondsensing probe to reflect light towards an output surface where theoptical detector is located. It is considered that the use of ananopillar as the outcoupling structure is a preferable option with oneor more spin defects located at a distal end of the nano pillar theproximal end of the nanopillar being attached to a micron scale diamondsupport. As described in more detail later, processing methodology hasbeen developed to fabricate a nanopillar into a high quality singlecrystal diamond support with a good quality spin defect located veryclose to the distal end of the nanopillar. The nanopillar can beprocessed to have dimensions suitable for waveguiding fluorescentemission from the spin defect(s) located therein as well as optimizingthe number of spin defects present within the nanopillar to achieve goodresolution. For example, the nanopillar may have a diameter between 100nm and 300 nm and a length between 0.5 μm and 5 μm. Furthermore, thenanopillar can be principally aligned along a crystallographic axis, thecrystallographic axis comprising one of: a <111> axis; a <110> axis; anda <100> axis. By the provision of an optical outcoupling structure asdescribed herein it is possible to achieve an optical collectionefficiency for the emitted fluorescent light between 0.01 to 0.10.Furthermore, it is possible to achieve a fluorescent photon count rateof greater than 50,000 counts/s, 100,000 counts/s, 150,000 counts/s,200,000 counts/s, 250,000 counts/s, or 300,000 counts/s

The mounting system is advantageously configured to position the one ormore spin defects within a few nanometers of a sample and scan across asample surface. For example, the mounting system may comprise an AFM(atomic force microscope). An optical microscope can be coupled to themounting system and configured to optically address and readout the oneor more spin defects. For example, the optical microscope may be aconfocal microscope that is integrated with an AFM.

The system may further comprise a further source of electromagneticradiation to manipulate the spin state of the one or more spin defects.For example, a microwave source, may be configured to generatemicrowaves tuned to a resonant frequency of the one or more spindefects. When the one or more spin defects are NV defects, the systemcan be configured to detect an external magnetic field by measuring aZeeman shift of spin states in the V defects via microwave manipulationof the spin defects in combination with fluorescence detection. Tofurther improve spin coherence, and thus sensitivity, the microwaves maycomprise a spin-decoupling sequence of pulses, wherein the sequenceincludes at least one of: a Hahn spin-echo pulse sequence; a CPMG (CarrPurcell Meiboom Gill) pulse sequence; an XY pulse sequence; and a MREVBpulse sequence.

Utilizing the aforementioned methodology it is possible to configure thesystem to have an AC magnetic field detection sensitivity better than200, 100, 75, 60, 50, 25, 10, or 5 μT Hz^(−1/2) (e.g. at frequenciesbetween 33 kHz and 10 MHz) and a DC magnetic field detection sensitivitybetter than 50, 20, 10, 6, 4, 1, or 0.5 μT Hz^(−1/2). Furthermore, it ispossible to configure the system to resolve single spin defects in asample. Further still, due to the improvements described herein it ispossible to significantly reduce the required integration time forsingle spin imaging while simultaneously achieving a good high signal tonoise ratio, e.g. a signal to noise ratio of at least 2 with anintegration time of no more than 10 mins (minutes), 5 mins, 3 mins, 2mins, 1 min, 30 seconds, 15 seconds, 10 seconds, 5 seconds, 2 seconds, 1second, or 0.5 second.

While high sensitivity magnetometry can be achieved as indicated above,electric field sensing and temperature sensing are also envisaged. Forexample, where the one or more spin defects are NV defects, the systemcan be configured to detect an external electric field by measuring aStark shift caused by the external electric field mixing m_(s)=+1 andm_(s)=−1 states in the NV defects. Alternatively or additionally, thesystem can be configured to detect a temperature using the NV defects,by measuring variation of axial zero-field splitting (ZFS) parameter ofthe NV centres. Temperature detection can also be used to calibrate outthe effects of temperature on magnetic field sensing. For example, thesystem can be configured to monitor a temperature of the diamondmaterial of the sensing probe so that one or more temperature-dependenteffects on the detection of the magnetic field can be calibrated out.Alternatively, both m_(s)=+1 and m_(s)=−1 resonances can be measured soas to provide a feedback mechanism for calibrating out the one or moretemperature-dependent effects. As such, the system can be configured toprovide magnetic field measurements that are unaffected by temperatureby using the m_(s)=±1 resonance transitions

Embodiments of the present invention also provide methods of usingsystems such as described herein. Such methods comprise: movablypositioning an optical outcoupling structure of a sensing probe withrespect to a surface of a sample; wherein the optical outcouplingstructure contains one or more spin defects, and is configured tooptically guide fluorescent light emitted by the spin defects toward anoutput end of the optical outcoupling structure; irradiating the spindefects with excitation light and microwaves so as to cause the spindefects to emit fluorescent light; and detecting the emitted fluorescentlight that exits through the output end of the optical outcouplingstructure after the fluorescent light has been optically guided throughthe optical outcoupling structure. Such methods may further comprisescanning the sample surface while maintaining a desired distance betweenthe spin defects and the sample surface, so as to obtain informationabout the sample surface. The provision of a movable optical outcouplingstructure in combination with one or more spin defects which can belocated in a controlled manner close to a sample enables sensing withhigh sensitivity, high resolution, and low data acquisition times.

In addition to the above, it has been found that the one or more spindefects can be reliably and accurately positioned close to a sensingsurface by utilizing a method comprising: movably positioning an opticaloutcoupling structure of a sensing probe with respect to a samplewherein the optical outcoupling structure contains a spin defectconfigured to emit fluorescent light in response to excitation lightfrom an optical source and microwaves from a microwave source; andwherein the optical outcoupling structure is configured to opticallyguide fluorescent light emitted by the spin defect toward an output endof the optical outcoupling structure; measuring a distance between thespin defect and the sample; etching a distal end of the opticaloutcoupling structure so as to reduce the distance between the spindefect and the sample; and repeating the acts of measuring said distanceand etching said distal end of said optical outcoupling structure, untilthe distance has been reduced by a desired amount.

Further still, it has been found that contamination of a sensing probeduring use can lead to a reduction in performance. As such, amethodology has been developed to clean the sensing probe to retain highperformance. Such a method comprises: providing a sensing probe formedof a diamond material comprising one or more spin defects configured toemit fluorescent light, the sensing probe further comprising a diamondnanopillar configured to optically guide the fluorescent light emittedby the spin defect toward an output end of the nanopillar; scanning,without AFM feedback, a sharp tip of a sample with the nanopillar, bymoving the diamond nanopillar with respect to the sample tip; andrepeating the act of scanning, without AFM feedback, the sharp tip ofthe sample with the nanopillar, until contamination of the nanopillar isreduced by a desired amount.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings disclose illustrative embodiments. They do not set forthall embodiments. Other embodiments may be used in addition or instead.

FIG. 1A illustrates an NV centre consisting of a substitutional nitrogenatom neighboring a lattice vacancy.

FIG. 1B shows the electronic structure of an NV centre in diamond.

FIG. 2 is a schematic block diagram of a nanoscale scanning NV sensorsystem that implements topside collection, in accordance with one ormore embodiments of the present application.

FIG. 3A shows a representative SEM (scanning electron microscope) imageof a single-crystalline diamond scanning probe containing a single NVcentre within 25 nm of its tip, within an array of diamond platformswith nanopillars.

FIG. 3B is a confocal image of red: fluorescence from asingle-crystalline diamond probe.

FIG. 3C illustrates photon autocorrelation measurement for NVfluorescence.

FIG. 3D illustrates optically detected ESR that identifies the singleemitter in the nanopillar as an NV centre.

FIG. 3E illustrates spin-echo measurements for the NV centre in thediamond nanopillar device.

FIG. 4A schematically illustrates the fabrication of a diamondnanopillar, in accordance with one or more embodiments of the presentdisclosure.

FIG. 4B shows an SEM image of the resulting array of diamond platformswith nanopillars.

FIG. 5A shows an NV magnetic-field image of bit tracks on a magneticmemory.

FIG. 5B shows a similar magnetic image as in FIG. 5A, but with theNV-sample distance decreased by an estimated 50 nm.

FIG. 5C shows optically detected ESR of the sensing NV centre.

FIG. 5D is a line cut along white line shown in FIG. 5B.

FIG. 5E shows a model calculated NV response for the experimentalsituation of FIG. 5A, under the assumption of a simplified magneticsample.

FIG. 5F illustrates magnetic images as in FIG. 5A and FIG. 5B with anexperimental realization in which the smallest observed domains haveaverage sizes of 25 nm.

FIG. 6A is a schematic diagram of an experimental configuration forscanning of a diamond pillar over a sharp metallic tip, as well as theresulting fluorescence signal.

FIG. 6B shows a zoomed-in image of a red square region at the locationof the NV centre.

FIG. 6C is an AFM (atomic force microscopy) topography image obtainedsimultaneously with the data of FIG. 6B.

FIG. 7A shows the current distribution used to simulate magnetic bitsthat were imaged with an NV scanning sensor.

FIG. 7B shows the magnetic field, projected on the NV axis, generated bythe current distribution in FIG. 7A.

FIG. 7C illustrates the NV magnetometry response obtained from themagnetic field distribution shown in FIG. 7B.

FIG. 8A shows the total NV fluorescence as a function of sample positionfor an NV in close proximity to the hard-disc sample.

FIG. 8B shows an NV magnetic image recorded simultaneously with the NVfluorescence counts of FIG. 8A.

FIG. 8C shows a line cut along the white line shown in FIG. 8A, averagedover 7 adjacent pixels.

FIG. 8D is a fluorescence approach curve on the magnetic memory medium.

FIG. 8E shows magnetic imaging with the same NV sensor that was used forFIGS. 8A-8D, and with a same experimental realization as in FIG. 5F.

FIG. 9A is an AFM image of the end of a scanning diamond nanopillarafter contamination during scanning.

FIG. 9B is an AFM image of the same nanopillar as in FIG. 9A, aftercleaning of the pillar's end face.

FIG. 10 is an SEM (scanning electron microscopy) image of a diamondnanopillar having a sharp pointed tip in accordance with one or moreembodiments of the present disclosure.

FIG. 11A illustrates AFM topography recorded during the experimentpresented in FIGS. 6A-6C.

FIG. 11B illustrates an approach-curve of far-field NV fluorescence rateas the nanopillar containing the NV centre is approached to the sample.

FIG. 11C shows a total fluorescence image reconstructed from thedatasets taken in conjunction with FIGS. 11A and 11B.

FIG. 11D illustrates measured NV fluorescence.

FIGS. 12A, 12B, 12C, 12D, 12E, and 12F illustrate alternativeembodiments of nanosensors based on spin defects.

FIGS. 13A-13G illustrate back-etching that allows the NV distance to befurther reduced.

DETAILED DESCRIPTION

Illustrative embodiments are discussed in this application. Otherembodiments may be used in addition or instead.

It should be understood that the present application is not limited tothe particular embodiments described, as such may vary. Also, theterminology used herein is for the purpose of describing particularembodiments only, and is not intended to be limiting, since the scope ofthe present application will be limited only by the appended claims.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although any methods andmaterials similar or equivalent to those described herein can also beused in the practice or testing of concepts described in the presentapplication, a limited number of the exemplary methods and materials aredescribed herein.

Where a range of values is provided each intervening value, to the tenthof the unit of the lower limit unless the context clearly dictatesotherwise, between the upper and lower limit of that range and any otherstated or intervening value in that stated range is encompassed withinthe invention. The upper and lower limits of these smaller ranges mayindependently be included in the smaller ranges is also encompassedwithin the invention, subject to any specifically excluded limit in thestated range. Where the stated range includes one or both of the limits,ranges excluding either or both of those included limits are alsoincluded in the invention.

In the present disclosure methods and systems are described relating tonanoscale scanning sensors that are based on spin defects, for exampleNV centres. In some embodiments the nanoscale scanning probes implementtopside collection in conjunction with combined AFM and opticalmicroscope.

Nanoscale sensing based on spin defects such as NV-centres is possiblebecause the NV-centre forms a bright and stable single-photon source foroptical imaging and has a spin-triplet ground state that offersexcellent temperature, magnetic and electric field sensing capabilities,as described in further detail below. Note that in the rest of thisdocument the negative charge state of the NV defect will be denotedsimply as NV. Factors that contribute to the excellent performance ofthe NV centre in such spin-based sensing schemes include, withoutlimitation, long NV spin coherence times, and efficient optical spinpreparation and readout. These properties persist from cryogenictemperatures to ambient conditions a feature that distinguishes the NVcentre from other systems that have been proposed as quantum sensors,such as single molecules or quantum dots.

FIG. 1A illustrates an NV centre consisting of a substitutional nitrogenatom 120 neighboring a lattice vacancy 110. As seen in FIG. 1A, the NVcentre is an empty position or vacancy 110, resulting from a missingcarbon atom 130 in the diamond lattice. An NV centre is relativelyinsulated from magnetic interference from other spins. The quantum stateof the spin of the NV centre may be probed and controlled at roomtemperature. NV centres in diamond, as well as systems involving othertypes of defects in solid state lattices, can provide electronic spinsthat have very little interaction with the background lattice. Suchelectronic spins are optically detectable with unique opticalsignatures. NV centres are visible as red spots when illuminated bylaser.

FIG. 1B shows the electronic structure of an NV centre in diamond. Asseen in FIG. 1B, the NV centre's ground state is paramagnetic and a spinone (S=1) triplet. The ground state of the NV centre is split intom_(s)=0 and doubly degenerate m_(s)=±1 sublevels, with a Δ=2.87 GHzcrystal field splitting. The NV centre emits fluorescent radiation fromits transitions between the electronic ground and excited states. Theelectronic transition is spin preserving, and the m_(s)=0 transition isbrighter than the m_(s)=±1 transition. Microwave excitation at theresonance frequency causes a fluorescence drop allowing the resonancefrequency to be measured by fluorescence measurement.

A static external field causes a Zeeman shift between m_(s)=+1 andm_(s)=−1 states, which is determined by a gyromagnetic ratio γ=2.8MHz/G. The degeneracy of the m_(s)=±1 states is thus lifted, under theexternal field, and the electron paramagnetic resonance spectrumcontains two resonance lines, one shifted to the higher and the othershifted to the lower frequency. By measuring the two shifted resonancefrequencies and their difference 6 w, the magnitude of the externalfield can be calculated.

As well as being able to detect magnetic fields using the NV defect ithas also been shown that it can be used to measure temperature (Phys.Rev. X 2, 031001 (2012)) and electric fields (Nature Physics Volume 7,Pages 459-463 (2011)). This could have a wide range of uses whencombined with certain embodiments of the present invention. For example,detection of the electric field, magnetic field and temperature in abiological specimen.

It is known in the art that the axial zero-field splitting (ZFS)parameter, D (2.87 GHz), varies significantly with temperature whichprovides a technical challenge for room-temperature diamond magnetometryas described in (Phys. Rev. B 82, 201202(R) (2010)). This property canbe used in order to deduce temperature i.e. in this invention anano-scale temperature sensor. For accurate B-field sensing it may bedesirable circumvent this temperature effect. Such methods mightinclude: 1) a temperature sensor to monitor the temperature of thediamond which the allows the effect of temperature to be calibrated out;2) Measurement of both the m_(s)=±1 resonances to provide a feedbackmechanism for controlling this; and 3) Use of the m_(s)=+1 resonancestransitions to provide the magnetic field measurement (opposed to thetransition between the m_(s)=0 and m_(s)=±1) which are unaffected bytemperature.

These methods to negate the effects of temperature variances duringmeasurement combined with certain embodiments of the present inventionallow for an improved sensing device.

An electric field at the NV centre can mix the m_(s)=+1 and m_(s)=−1states, causing a shift in its ZFS. Under the presence of small magneticfields (compared to the NV centre's strain), this shift comprises a(linear) Stark shift, which can be measured with high sensitivity. Forthis particular embodiment, AC electric field sensitivities ranging from50 to 200 Vcm⁻¹Hz^(−1/2) are achievable. For sensitivities in this range(e.g. 120 Vcm⁻¹Hz^(−1/2)), within 1 second of integration time, a fieldequivalent to 0.01 electron charges at a distance of 10 nm could bedetected (signal to noise of 1).

Variations in temperature at the NV centre induce changes in the latticeconstant which modify the local crystal field at the NV centre and shiftthe ZFS. Thus, by measuring the ZFS the NV centre additionally canoperate as a sensitive thermometer. For this specified embodiment, thetemperature can be measured with a sensitivity ranging from 0.2 to 0.8KHz^(−1/2).

Due to the above-described optical and magnetic properties of NVcentres, NV centres can be used as sensor probes. In order for an NVcentre to be a useful probe, the NV centre must be positioned withnanometer accuracy while simultaneously its fluorescence is beingmeasured. This can be achieved by using NV centres as sensor probes inconjunction with AFM methods combined with optical microscopy.

While some success has been achieved at implementing scanning NV sensorsusing diamond nanocrystals grafted onto scanning AFM probe tips, theseapproaches suffer from the poor sensing performance of nanocrystal-basedNV centres, for which the spin coherence times are typically orders ofmagnitude shorter than for NV centres in bulk diamond.

In the present application NV sensors are disclosed that use a diamondnanopillar as the scanning probe of an AFM, with an individual NV withina few nanometers of its distal end.

The NV might be placed through one or some combination of implantation,as grown, formed through irradiation and annealing.

FIG. 2 is a schematic block diagram of a nanoscale scanning NV sensorsystem 200 that allows for topside collection, in accordance with one ormore embodiments of the present application. The system 200 includes amonolithic scanning NV sensor that uses a diamond nanopillar 210 as thescanning probe, with an individual NV centre 208 artificially created ata small distance, for example about 25 nm, from a distal end of thediamond nanopillar.

The system 200 includes a combined AFM 202 and optical microscope, i.e.an AFM 202 that is used together with the optical microscope. In someembodiments, the AFM 202 and the optical microscope may be integratedinto a single instrument. In the illustrated embodiment, the opticalmicroscope is a confocal microscope including a confocal microscopeobjective 222.

The AFM 202 includes a diamond nanopillar 210 attached to a diamondcantilever 203 of the AFM 202. The diamond cantilever 203 may beattached to a positioning system 207, which for example may include amoveable stage on which a sample 209 can be placed, as shown in FIG. 2 .The diamond nanopillar 210 is thus movably positioned relative to asurface of the sample 209, so that the sample surface can be scanned bythe diamond nanopillar 210 using AFM feedback. In some embodiments, thepositioning system 207 may include 3-axis piezoelectric positionersconfigured to position the sample and the AFM head with respect to afixed optical axis of the optical microscope.

In some embodiments, micro-meter thin, single-crystalline diamond slabs215 are fabricated to produce the diamond nanopillar 210. As describedin further detail in conjunction with FIG. 4A below, the diamondnanopillar 210 can be fabricated so as to contain a single NV centre208, which can be brought within a few tens of nanometers of targetsamples 209.

The diamond nanopillar 210 has an elongated configuration and includes adistal end 214 that can be positioned directly opposite a samplesurface, and a proximal end 216 to be coupled to the AFM cantilever 203.The diamond nanopillar 210 is thus configured to optically guide thefluorescence emitted by the NV centre 208, in a direction generallyextending from the distal end 214 toward the proximal end 216.

In the illustrated embodiment, the size of the diamond nanopillar 210 isabout 200 nm in diameter, which optimizes the above-described opticalwaveguiding by the diamond nanopillar. Other embodiments may usedifferent sizes for the diamond nanopillar 210, for example nanopillarsthat are less than 200 nm in diameter. In the illustrated embodiment,the size of the diamond cantilever 203 may be about 4 μm×30 μm inlateral dimensions, and about 2 μm in height. Other embodiments may usedifferent sizes for the diamond cantilever for example diamondcantilevers that have larger, or smaller, lateral dimensions and/orheight.

The system 200 further includes an optical source 220 configured togenerate excitation light that causes emission of fluorescent light fromthe color centres when applied thereto. In the illustrated embodiment,the optical source 220 is a 532-nm excitation laser. In otherembodiments, different types of optical sources can be used. Forexample, the optical source may be a laser or an LED tunable to awavelength less than 637 nm.

The microwave source 230 is configured to generate pulses of microwaveradiation and to apply the microwave pulses to the NV centre 208. Insome embodiments the microwave source 230 is configured to apply to theNV centre microwave pulses tuned at the resonance frequency of the NVcentre 208, during excitation of the NV centre by the laser light fromthe laser 220, and during scanning of the sample surface by the diamondnanopillar 210.

The microwave source may be configured to apply to the NV centremicrowave pulses that allow for precession of the electronic spin of theNV centre about an external magnetic field to be sensed, the frequencyof the precession being linearly related to the magnetic field by theZeeman shift of the energy levels of the electronic spin, so that astrength of the external magnetic field can be determined from themeasured Zeeman shift.

The microwave source may be configured to apply to the NV centremicrowave pulses so that a strength of the external electric field canbe determined from a measured shift in the energy levels of theelectronic spin of the NV centre.

In some embodiments of the present application topside collection isimplemented. In these embodiments, the optical fluorescence from the NVcentre 208 is read out from the topside of the cantilever 203 and thusthrough the entire nanopillar 210. In other words, the fluorescenceemitted by the NV centre 208 is detected by the optical detector 240after the fluorescent light from the NV centre 208 has been opticallyguided throughout the length of the diamond nanopillar 210 and exitsthrough the proximal end 216 of the diamond nanopillar 210.

Topside readout allows for significantly improved performance of the NVsensor system 100. For example, topside readout al lows the study ofsamples that are non-transparent to the fluorescence radiation from theNV centre 208. If bottom-side readout were used, as has beenconventionally done, then the fluorescence would be collected throughthe sample itself, which severely limits the type of samples which canbe studied.

Moreover, topside readout takes advantage of optical waveguiding fromthe diamond nanopillar 210 to enhance photon collection efficiency. Insome embodiments, the collection efficiency may be improved by a factorof about 5. Such increased collection efficiency leads to an increasedsensitivity, which is an important advantage in many applications.

By implementing the above-described topside collection using diamondnanopillars as described above, the excitation light, typically greenlaser at 532 nm, is better isolated from the samples. This can beimportant for imaging samples which react negatively to intense laserfields, for example biological samples. The amount of required power isminimized to saturate the NV fluorescence due to the above-describedoptical waveguiding by the diamond nanopillar 210. In some embodiments,there is a reduction of a factor of about 10 in saturation power. Inaddition, as the excitation light from the laser is focused on thewaveguiding mode of the nanopillar far-field excitation of the samplecan be minimized.

In these embodiments, the system 200 includes at least one opticaldetector 240 that is positioned so as to receive the emitted fluorescentlight that exits through the proximal end 216 of the diamond nanopillar210, after being optically guided through the length of the diamondnanopillar as described above. Because diamond nanopillars are efficientwaveguides for the NV fluorescence band, very high NV signal collectionefficiencies can be achieved using the above-described topsidecollection method.

In these embodiments, the microscope objective 222 is disposed betweenthe optical detector 240 and the topside of the cantilever 203, as seenin FIG. 2 . The objective 222 may be a long-working-distance microscopeobjective 222. In some embodiments, the objective 222 may have anumerical aperture of about 0.7. Other embodiments may use microscopeobjectives with numerical apertures that are greater or less than 0.7.

The system 200 may also include one or more dichroic mirrors thatseparate the fluorescence emitted by the NV centre 208 from theexcitation light generated by the laser 220.

The AFM 202 may include an AFM feedback system configured to control thedistance between the NV centre and the sample surface. The spatialresolution of an NV sensor is affected by the distance from the NVcentre to the sample. Proper AFM control, including without limitationmechanical control and feedback control, must be achieved to assureclose proximity of the NV centre to the sample surface. Bad mountingand/or improper AFM feedback control can lead to excessive AFMtip-sample distances. In some embodiments an AFM feedback system, forexample provided by an Attocube ASC500 controller, may be used to propersetup and tuning of AFM feedback, which in conjunction with accuratemounting of AFM tips allows for precise observation of desiredvariables.

A processing system may be integrated with the system described above,and is configured to implement the methods, systems, and algorithmsdescribed in the present application. The processing system may include,or may consist of, any type of microprocessor, nanoprocessor, microchip,or nanochip. The processing system may be selectively configured and/oractivated by a computer program stored therein. It may include acomputer-usable medium in which such a computer program may be stored,to implement the methods and systems described above. Thecomputer-usable medium may have stored therein computer-usableinstructions for the processing system. The methods and systems in thepresent application have not been described with reference to anyparticular programming language; thus it will be appreciated that avariety of platforms and programming languages may be used to implementthe teachings of the present application.

FIG. 3A shows a representative scanning electron microscope (SEM) imageof a single-crystalline diamond scanning probe containing a single NVcentre within ˜25 nm of its tip. To prepare such devices a series offabrication steps are performed sequentially including forming NVcreation (e.g. through low-energy ion implantation), severalsuccessively aligned electron-beam lithography steps and reactive ionetching.

An important element to this sequence is the fabrication ofmicrometre-thin single-crystalline diamond slabs that form the basis ofthe scanning probe device shown in FIG. 3A.

The scanning diamond nanopillars have typical diameters of ˜200 nm andlengths of ˜1 μm and are fabricated on few-micrometre-sized diamondplatforms that are individually attached to atomic force microscope(AFM) tips for scanning.

FIG. 3B shows a confocal image of red fluorescence from asingle-crystalline diamond probe whereas FIG. 3C illustrates photonautocorrelation measurement for the NV fluorescence. In FIG. 3B, aconfocal scan was performed of a typical single scanning NV device,under green laser illumination, at an excitation wavelength of 532 nm.The bright photon emission emerging from the nanopillar (white circle)originate from a single NV centre, as indicated by the pronounced dip inthe photon-autocorrelation measurement shown in FIG. 3B, and thecharacteristic signature of optically detected NV electron-spinresonance (ESR) seen in FIG. 3C. These results, all obtained from a samedevice, confirm that photon waveguiding through the nanopillar persistsdespite the close proximity of the NV to the tip of the fabricatednanopillar devices.

FIG. 3D illustrates optically detected ESR that identifies the singleemitter in the nanopillar as an NV centre. The data in FIG. 3D wereobtained at 100 μW excitation power and demonstrate single NV countsapproaching 2.2×10⁵ counts per second (c.p.s.)—an approximately fivefoldincrease in detected fluorescence intensity compared to an NV observedunder similar conditions in an unpatterned diamond sample. Thus, thereis a significant increase in fluorescence signal strength from thescanning NV and at the same time minimal exposure of the sample to greenexcitation light. This may be particularly relevant for possiblebiological or low-temperature applications of the scanning sensor.

FIG. 3E illustrates spin-echo measurements for the NV centre in thediamond nanopillar device. The spin coherence time T₂ of the NV centremay be characterized using well-established techniques for coherentNV-spin manipulation. Spin-coherence sets the NV sensitivity to magneticfields and limits the number of coherent operations that can beperformed on an NV spin; it is therefore a figure of merit forapplications in magnetic-field imaging and quantum informationprocessing.

Using a Hahn-echo pulse sequence, the characteristic single NV coherencedecay shown in FIG. 3 was measured. From the decay envelope a spincoherence time of T₂=74.8 μs was deduced. This T₂ time is consistentwith the density of implanted nitrogen ions (3×10¹¹ cm⁻²) and it can beconcluded that the device fabrication procedure fully preserves NV spincoherence. Combining measurements of the T₂ time with the fluorescencecount rate and NV spin readout contrast as obtained above a maximal a.c.magnetic field sensitivity of 56 nT Hz^(−1/2) at a frequency of 33 kHzand, based on data in FIG. 30 a d.c. sensitivity of 6.0 μT Hz^(−1/2) wasobtained.

Both a.c. and d.c. magnetic field sensitivities may be further improvedby using spin-decoupling sequences and optimized parameters for spinreadout respectively. In some embodiments, ranges between about 10 nTHz^(−1/2) and 100 nT Hz^(−1/2) may be attained for the a.c. magneticfield sensitivities. In some embodiments, a.c. sensitivities better than200, 100, 75, 60, 50, 25, 10, or 5 nT Hz^(−1/2) may be attained.

In some embodiments ranges between about 0.5 μT Hz^(−1/2) and 10 μTHz^(−1/2) may be attained for the d.c. magnetic field sensitivities. Insome embodiments d.c. sensitivities better than 50, 20, 10, 6, 4, 1, 0.5μT Hz^(−1/2) may be attained.

FIG. 4A schematically illustrates the fabrication of a diamondnanopillar that is used as an AFM probe tip in accordance with one ormore embodiments of the present application. In overview, electron-beamlithography is used to define nanopillars and platforms from the top andbottom sides of the diamond membrane 414. Patterns are then transferredto the diamond by RIE (reactive ion etching). Mask deposition andtop-etch results in an intermediate structure 410, which in theillustrated embodiment has a SiO₂ mask. Mask deposition and bottom-etchresults in a structure 420 which allows creation of single NV centres416 in each individual nanopillar 408 through ion implantation. Themonolithic diamond membrane 414 may have a thickness on the order of afew micrometers.

In some embodiments, the structures described in the above paragraph maybe fabricated from a sample of high-purity, single-crystalline diamond,which for example may be electronic grade diamond from Element Six. Inthe case of single crystal, the major face, i.e. that perpendicular tothe eventual direction of the nano-pillar might be substantially (lessthan 10, 5, 2 degrees) from the principal crystallographic axes of{111}, {110} or {100}. In some cases, polycrystalline or HPHT typediamond might also be suitable.

In one example the sample may be bombarded with atomic nitrogen at anenergy of about 6 keV and a density of about 3×10¹¹ cm⁻² leading to anominal mean NV depth of about 10 nm. Subsequent annealing at about 800C for about 2 hours may yield a shallow layer of NV centres with adensity of ×25 NVs/μm² and a depth of about 25 nm.

In these embodiments, the sample is then etched from the non-implantedside to a thickness of about 3 μmusing reactive ion etching. In someembodiments, a cyclic etching recipe may be used that includes a 10 minArCl₂ etch, followed by 30 min of O₂ etching and a cooling step of 15minutes. This sequence allowed the integrity of the diamond surface tobe maintained during the few-hour etching time. On the resulting thindiamond membrane 414, an array of diamond nanopillars 210 can befabricated on the top side by using electron-beam lithography and RIE asdescribed above.

Next, a second lithography step can be performed on the back-side of thediamond membrane, which defined platforms to hold the diamondnanopillars. A final RIE process transferred the resist pattern to thesample, and fully cut through the diamond membrane to yield thestructure shown in FIG. 4A. FIG. 4B shows an SEM image of the resultingarray of diamond platforms with nanopillars.

In a second example the NVs might be produced using growth alone, orthrough conversion of N to NV through using known methods in the art ofirradiation (e.g. electron irradiation) and annealing.

In some embodiments, the nanoscale NV sensor systems described above canbe applied to nanoscale magnetic sensing and/or imaging. For example, inone or more embodiments the system 100 may be used to sense the magneticfield generated by the spins contained in the sample 209 scanned by thediamond nanopillar 210. Standard spin-echo techniques may be used forcoherent NV-spin manipulation. In these embodiments, the microwavesource 230 is configured to apply to the NV centre microwave pulses thatcause the spin of the NV centre to process under the influence of aZeeman shift, so that a strength of an external field can be determinedfrom the measured Zeeman shift. For applications such as magneticimaging, the microwave source 230 may be configured to apply themicrowave pulses to the NV centre 208 while the diamond nanopillar 210is scanning the sample surface, so that a magnetic field image of thesample surface can be obtained by the system 200.

Using well-established techniques for coherent NV-spin manipulation asmentioned above, the spin coherence time T₂ of the NV centre can becharacterized. Spin-coherence sets the NV sensitivity to magnetic fieldsand limits the number of coherent operations that can be performed on anNV spin. Spin coherence is thus an essential figure of merit forapplications in magnetic field imaging and quantum informationprocessing.

To obtain information about single NV coherence decay, a Hahn-echo pulsesequence can be used. In this way, a spin coherence time of T₂=74.9 μsis obtained for the diamond nanopillar described in conjunction withFIG. 2 above. Examples of other decoupling pulse sequences that can begenerated by the microwave source 230 include without limitation: a CPMG(Carr Purcell Meiboom Gill) pulse sequence; an XY pulse sequence; and aMREVB pulse sequence.

Combining measurements of the T₂ time with measured values of thefluorescence count rate and NV spin readout contrast, an AC magneticfield sensitivity of about 56 nT Hz^(−1/2) at a frequency of about 33kHz, and a DC magnetic field sensitivity of about 6.0 μT Hz^(−1/2) canbe obtained for the embodiments described above. Both AC and DC magneticfield sensitivities can be further improved by using spin-decouplingsequences and/or optimized parameters for spin readout.

FIGS. 5A-5E illustrate methods and results relating to the imaging of ananoscale magnetic memory medium characterize the resolving power of thescanning NV sensor. A nanoscale magnetic memory medium, consisting ofbit tracks of alternating (out-of-plane) magnetization with various bitsizes was imaged. The scanning NV sensor operated in a mode that imagedcontours of constant magnetic field strength (B_(NV)) along the NV axisthrough the continuous monitoring of red NV fluorescence, in thepresence of an ESR driving field of fixed frequency ω_(MW) and typicalmagnitude B_(MW)≈2 G, as determined from NV Rabi oscillations (notshown). ω_(MW) was detuned by δ_(MV) from the bare NV spin transitionfrequency ω_(MW), but local magnetic fields due to the sample changedthis detuning during image acquisition. In particular, when local fieldsbrought the spin transition of the NV into resonance with ω_(MW) a dropin NV fluorescence rate was observed, which in the image yielded acontour of constant of B_(NV) δ_(MW)/γ_(nv), with γ_(NV)=2.8 MHzG⁻¹being the NV gyromagnetic ratio.

FIG. 5A shows a resulting scanning NV magnetometry image of two stripesof magnetic bits, indicated by the white dashed lines with bit spacingsof 170 nm and 65 nm. The normalized data, I_(norm)=I_(RF,1)/I_(RF,2),was plotted, to reveal magnetic field lines corresponding to a samplemagnetic field along the NV axis of B_(NV)=±3 G. Additionally, a biasmagnetic field of B_(NV)≈52 G was applied to determine the sign of themeasured magnetic fields. The shape of the observed domains is wellreproduced by calculating the response of the NV magnetometer to anidealized sample with rectangular magnetic domains of dimensionscorresponding to the written tracks.

FIG. 5B shows a similar magnetic image as in FIG. 5A, but with theNV-sample distance decreased by an estimated 50 nm. The spatialresolution of an NV magnetometer is affected by the distance from the NVcentre to the sample. Bringing the NV closer to the sample increases themagnetic field magnitude at the NV sensor, and improves the imagingspatial resolution allowing the imaging of magnetic bits×38 nm in width.Approaching the NV sensor more closely to the magnetic sample revealedmagnetic bits with average sizes of ˜28 nm, as shown in FIG. 4B. In thisimage due to the large field gradients generated at the boundariesbetween domains, transitions between magnetic field lines could beobserved on length scales of ˜3 nm. Approximate NV-sample distances arenoted in the schematics illustrating the experimental configuration,with the sensing NV centre fixed on the optical axis and the magneticsample scanned below the pillar. Total image acquisition times were 11.2min (50 ms per pixel) for FIG. 5A and 12.5 min (75 ms per pixel) fordata in FIG. 5B, with laser powers of 130 μW.

FIG. 5C shows optically detected ESR of the sensing NV centre. FIG. 5Dis a line cut along white line shown in FIG. 5B. FIG. 5B shows a modelcalculated NV response for the experimental situation of FIG. 5A, underthe assumption of a simplified magnetic sample. FIG. 5F illustratesmagnetic images as in FIG. 5A and FIG. 5B with an experimentalrealization in which the smallest observed domains have average sizes of25 nm.

An even further decrease of NV-sample distance enables imaging of yetsmaller domains, ˜25 nm in width (FIG. 5F), but with a reduced imagingcontrast caused by strong magnetic fields transverse to the NV axis,which occur in close vicinity to the surface of the sample. One of thedisadvantages of using a hard drive to characterize the tip is that thelocal magnetic fields are very large and exceed the typical dynamicrange of this technique. However, such experiments provide valuableinformation regarding NV-sample distance, and consequently the spatialresolution achieved in imaging. In particular, it is estimated that thedistance between the scanning NV and the sample to be comparable to 25nm, based on the smallest magnetic domain sizes observed.

To independently verify the proximity of the NV to the diamond surface,a measurement was conducted in which a sharp metallic tip (<20 nm indiameter) was scanned over the NV-containing pillar to image thelocation of the NV. The imaging contrast consisted of the detected NVfluorescence in the far-field changing when the NV was located in closeproximity to the metallic tip. Owing to the strong dependence of NVfluorescence rate on the distance between the NV and the metallicsample, in this case due to partial fluorescence quenching and localmodifications of excitation light intensity this technique could be usedto precisely locate the position of the NV centre within the diamondnanopillar.

FIGS. 6A, 6B, and 6C illustrate nanoscale fluorescence quenching imagingof the scanning NV sensor. FIG. 6A is a schematic diagram of anexperimental configuration for a scanning of the diamond pillar over asharp metallic tip, as well as the resulting fluorescence signal. FIG.6B shows a zoomed-in image of a red square region at the location of theNV centre. FIG. 6C is an AFM topography image obtained simultaneouslywith the data of FIG. 6B.

Scanning the diamond pillar over a sharp metallic tip leads to a brightcircular feature due to the sample topography. Positioning the metallictip exactly at the location of the NV centre (shown in square), however,yields a sharp dip in NV fluorescence. The illustration shows theexperimental configuration used in this experiment.

FIG. 6B is a zoomed-in image of the square region in FIG. 6A. Theobserved fluorescence quenching dip has a spatial resolution of ×20 nm.FIG. 6C is an AFM topography image obtained simultaneously with the datain FIG. 6B. The scale bars represent 100 nm displacement in alldirections. Image acquisition times were 30 min (320 ms per pixel) and2.7 min (250 ms per pixel) in FIG. 6A and FIG. 6B, respectively, at alaser power of 35 μW.

The resulting data showed signatures of the topography of the scanningdiamond nanopillar, appearing as bright ring in the NV fluorescencesignal. More importantly, however, while the front-end of the diamondprobe scanned over the sharp metallic tip, a dip in NV fluorescence(square in FIG. 6A and zoomed image in FIG. 6B) was observed when themetallic tip was positioned at the location of the NV centre. Thisfeature is not accompanied by any topographic features and is thusattributed to partial quenching of the NV fluorescence due to the sharpmetallic tip. The Gaussian width (double standard deviation) of 25.8 nmof this fluorescence quenching spot was probably still limited by thesize of the metallic tip and therefore marks an upper bound to theability to localize the NV centre within the pillar. Such data allow theposition of the single NV centre to be found with respect to thetopography of the device. This may greatly facilitate precise alignmentof the sensing NV centre with respect to targets in future sensing andimaging applications.

A remaining uncertainty to the distance between the scanning NV centreand the sample is vertical straggle in the NV implantation process.Naturally occurring stable NV centres have been observed as close as 3nm from diamond surfaces, so future advances in the controlled creationof NV centres may allow for the NV-sample distance to be furtherimproved and therefore the spatial resolution in scanning NV imaging byabout one order of magnitude. Additionally, the coherence properties ofartificially created NV centres close to the diamond surface could befurther improved by annealing techniques or dynamical decoupling, whichmay both significantly improve the magnetic sensing capabilities of thescanning NVs. For magnetic field imaging, the ability to resolveindividual magnetic domains, using the above described methods, equalsthe typical performance of alternative methods, with the addedadvantages of being non-invasive and quantitative.

FIGS. 7A, 7B, and 7C relate to the simulation of magnetic imagesobtained with an NV scanning sensor. FIG. 7A shows the currentdistribution used to simulate magnetic bits that were imaged with an NVscanning sensor, whereas FIG. 7B shows the magnetic field, projected onthe NV axis, generated by the current distribution in FIG. 7A.

In order to reproduce the magnetic images obtained with the scanning NVsensor, a model-calculation of the local magnetic fields in proximity tothe hard-disc sampled imaged in the experiment was performed. Themagnetic domains can be approximated by an array of current-loops in thesample-plane as illustrated in FIG. 7A. The sizes of the loops can bechosen to match the nominal size of the magnetic bits on the sample,which has bit-width 200 nm and bit-length 125 nm and 50 nm for thetracks shown in the figure. The current was set to 1 mA, correspondingto a density of ˜1 Bohr magneton per (0.1 nm)². These values were foundto yield the best quantitative match to the magnetic field strengthsobserved in the experiment. Biot-Savart's law can then be applied to thecurrent-distribution to obtain the magnetic field distribution in thehalf-plane above the sample.

FIG. 7B shows the resulting magnetic field projection onto the NV centreat a scan height of 50 nm above the current loops. The NV direction canbe experimentally determined to be along the ([011]) crystallinedirection of the diamond nanopillar in a coordinate-system where x-, y-and z-correspond to the horizontal, vertical and out-of plane directionsin FIG. 7B, by monitoring the NV-ESR response to an externally appliedmagnetic field. The external magnetic field may be applied using 3-axisHelmholtz-coils. Slight variations of the NV orientation due toalignment errors between the diamond crystallographic axes and the scandirections can be allowed to find the NV orientation that reproduce theexperimental data best. With this procedure, the NV orientation, with(√{square root over (2)} sin(ϕ),√{square root over (2)} cos(ϕ),1)/√{square root over (5)}=π 162/180, was found.

This magnetic-field distribution can be used to calculate the responseof the NV centre to a magnetometry scan, as described above. For thiscalculation, Lorentzian ESR response with a full-width at half maximumof 9.7 MHz, a visibility of 20% and two external RF sources withdetunings±10 MHz from the bare ESR frequency were assumed, in accordancewith original experimental parameters.

FIGS. 8A, 8B, 8C, 8D and 8E relate to NV magnetometry in close proximityto a strongly magnetized sample.

The presence of a strong magnetic field B⊥, transverse to the NV axisleads to a reduction of contrast in optically detected ESR and moreoverreduces the overall fluorescence intensity of the NV centre. Theseeffects result from a mixing of the NV spin-levels in the optical groundand excited states of the NV centre in the presence of B⊥. Such mixingon one hand allows for spin non-conserving optical transitions and onthe other hand suppresses the spin-dependence in shelving from the NVexcited state (triplet) to the metastable NV singlet states. Bothspin-conservation under optical excitation and spin-dependent shelvingare responsible for the non-zero contrast in optically detected ESR ofNV centres and consequently, their suppression with transverse magneticfields explains the disappearance of NV magnetometry features whenclosely approaching a strongly magnetized sample.

FIG. 8A shows the total NV fluorescence as a function of sample positionfor an NV in close proximity to the hard-disc sample, while FIG. 8Bshows an NV magnetic image recorded simultaneously with the NVfluorescence counts of FIG. 8A. FIG. 8C shows a line cut along the whiteline shown in FIG. 8A, averaged over 7 adjacent pixels. In particular,the raw NV fluorescence counts in FIG. 8A were observed when scanning anNV in a diamond nanopillar in close proximity to the sample at anestimated distance of 25 nm between NV and sample surface. Dark featuresappear when the NV is scanned over magnetic bits that enhance B⊥, whilethe inverse happens when B⊥ is reduced, or the longitudinal field B_(NV)enhanced, by local fields. This mode of bit-imaging allows for spatialresolutions≈2: 0-30 nm, as seen in FIG. 8C.

At the same time, a magnetic image recorded with the technique describedin the main text shows no appreciable imaging contrast. FIG. 8D is afluorescence approach curve on the magnetic memory medium. FIG. 8E showsmagnetic imaging with the same NV sensor that was used for FIGS. 8A-8D.Only exceedingly long integration times on the order of hours wouldallow for weak magnetic features with dimensions on the order of 20 nmto be revealed, as seen in FIG. 8D.

The rates of the two effects which lead to a disappearance of ESRcontrast, i.e. spin-flip optical transitions and shelving of m_(s)=0electronic states into the metastable singlet, scale approximately as

${\left( \frac{B_{\bot}}{D_{GS} - D_{ES}} \right)^{2}{and}\left( \frac{B_{\bot}}{D_{ES}} \right)^{2}},$

respectively, with D_(GS(ES)) the ground- (excited-) state zero-fieldspin splitting of 2.87 GHz and 1.425 GHz respectively. Given thatD_(GS)≈2; D_(ES), the scaling of the two mechanisms with B⊥ will be verysimilar. The characteristic scale of D_(ES) (D_(GS)/2) for thedisappearance of ESR contrast thus allows us to estimate B⊥ close to thesample to be B⊥≈D_(ES)/γ_(NV)≈514 Gauss. However this may give anoverestimation of B⊥ as smaller values can already significantly affectESR contrast and NV fluorescence intensity due to the complex dynamicsof NV spin pumping. Indeed, strong reductions of NV fluorescence ratesfor B⊥ less than 100 G have been observed in the past. Transversemagnetic fields on this order were consistent with the larges on-axismagnetic fields observed in the above experiments as well as with thecalculations of magnetic field profiles described in this application.For the parameters used for these figures, maximal values of B⊥≈0.200Gauss for an NV-to-sample distance of 20 nm are obtained.

NV-sample distance is an essential parameter for the performance of ourmicroscope as it determines the overall resolving power with which weakmagnetic targets can be imaged. Three parameters that can affectV-sample distance include: depth of NV centres in the diamondnanopillars contamination of scanning diamond nanopillars; and AFMcontrol.

It is desirable to have NV centres controllably positioned and close tothe diamond surface, for example closer than 50 nm, 40 nm, 30 nm, 20 nm,10 nm, or 5 nm.

The depth of the NV centres created using ion implantation for example,can be controlled by the energy of the ions used for NV creation.However, the stopping of ions in matter is a random process and thedepth of the created NV centres therefore not perfectly well-defined.This straggle in ion implantation poses an intrinsic uncertainty to thedistance between the scanning NV and the end of the diamond nanopillar.For implantation energies of 6 keV (with implantation-depths of 1-nm) asused in this work, NV straggle has recently been shown to be as large as10-20 nm. Since straggle in NV implantation is hard to circumvent, it isessential for the future to develop techniques to preciselypre-determine the depth of a given sensing Vin a diamond nanopillar.This could be performed using recently developed nanoscale imagingmethods for NV centres, or by scanning the NV sensor over a well-definedmagnetic field source.

The depth of NV centres produced through growth might be controlled bythe time and duration of adding nitrogen dopant gas to the CVD diamondgrowth process.

FIG. 9A is an AFM image of the end of a scanning diamond nanopillarafter contamination during scanning. FIG. 9B is an AFM image of the samenanopillar as in FIG. 9A, after cleaning of the pillar's end face.

During scanning-operation, the scanning diamond nanopillar can gathercontamination from the sample or environment. An example for such acontaminated diamond-tip is shown in the AFM image shown in FIG. 9A,which was acquired with the scanning protocol employed in FIG. 10 ,using a sharp diamond tip as shown in FIG. 10 . Such contamination canartificially increase the distance of the scanning NV centre to thesample by several 10's of nm, as seen for example in FIG. 9A. To undocontamination of the diamond-tip after excessive scanning over dirtysamples, a “tip-cleaning technique” can allow a contaminated tip torevert to its initial, clean state as illustrated by the transition fromFIG. 9A to FIG. 9B. Tip cleaning may be performed by repeated scanningof the diamond nanopillar over the sharp diamond tip (shown in FIG. 9A)in the absence of AFM feedback. Such feedback-free scanning can partlyremove contamination from the diamond pillar, which after repeatedoperation leads to a clean device as the one shown in FIG. 9B.

With proper sample-cleaning, control over environmental conditions andoccasional “tip-cleaning” runs, adverse effects of tip-contamination canbe eliminated. This, together with the excellent photo-stability of NVcentres, then allows for long term operation of the scanning NV sensor.

Proper AFM control is necessary to assure close proximity of the NVcentre to the sample surface. It has been shown in the past that badmounting or improper AFM feedback control can lead to AFM tip-sampledistances in excess of 20 nm. Careful mounting of AFM tips and propersetup and tuning of AFM feedback, which in some embodiments may beprovided by an Attocube ASC500 controller, may therefore be essential toobserve e.g. the fluorescence quenching features.

For the experiment described earlier in conjunction with nanopillarswith a sharp tip, sharp diamond tips were fabricated and metal coated inorder to localize the NV in the scanning nanopillar through fluorescencequenching. Diamond tip fabrication was based on the nanofabricationtechniques that we already employed for the production of the scanningdiamond nanopillars described above. A type Ib diamond (Element six) waspatterned with circular etch-masks (flowable oxide, Fox XR-1541, DowCorning) of 100 nm diameter.

In order to obtain sharp diamond tips instead of cylindrical diamondnanopillars, the RIE etching recipe previously used can be modified:while the oxygen etching chemistry can be kept identical to pillarfabrication, the etching time can be significantly increased, such as tocompletely erode the etch mask on the diamond substrate. As a result,the etched diamond structures acquired the form of sharp tips as shownin the representative SEM image in FIG. 10 . Typical tip-radii were inthe range of 10 nm and tip lengths were on the order of 200 nm.

For the experiments described above the sharp diamond tips were thencoated with a thin metallic layer using thermal metal evaporation. Toavoid oxidation of the metal, gold can be chosen as the quenching metaland a chrome adhesion can be used between the gold and the diamond. Forthe tips employed in this work, 5 nm of gold and 5 nm of chrome areused.

In some embodiments one of which is illustrated in FIG. 10 diamondnanopillars that have distal ends with a sharp pointed tip 410, ratherthan a cylindrical cross-section, may be fabricated. In theseembodiments, a diamond membrane can be patterned with circularetch-masks having a diameter on the order of about 100 nm. To obtain asharp diamond tip, the etching time is increased significantly so as tocompletely erode the etch mask on the diamond membrane 414. As a result,the etched diamond structures may acquire the form of a sharp tip 410 asshown in FIG. 10 .

While nanoscale scanning NV sensors have been described that use adiamond nanopillar as the scanning probe, with an individual NV centreat a small distance from a distal end of the diamond nanopillar manyother variations and embodiments are possible.

In general, a nanoscale scanning sensor system may include a solid statespin defect (for example the above-described NV centre in diamond),configured to emit fluorescent light in response to excitation lightfrom an optical source and microwave pulses from a microwave source. Thesystem may further include an optical outcoupling structure containing,or coupled to, the spin defect. The optical outcoupling structure may beconfigured to optically guide the fluorescent light emitted by the spindefect toward an output end of the optical outcoupling structure.

An optical detector may be configured to detect the fluorescent lightthat is emitted from the spin defect and that exits through the outputend of the optical outcoupling structure after being optically guidedtherethrough. A mounting system (for example an AFM) may be configuredto movably hold the optical outcoupling structure so as to control adistance between the spin defect and a surface of a sample whilepermitting relative motion between the optical outcoupling structure andthe sample surface. An optical microscope may be coupled to the mountingsystem and configured to optically address and readout the spin defect.

The optical outcoupling structure may be movably positionable relativeto the sample surface so that the sample surface can be scanned by theoptical outcoupling structure while the excitation light and themicrowaves are being applied to the spin defect.

In some embodiments, the optical outcoupling structure may be asingle-crystal diamond membrane. In some embodiments, the opticaloutcoupling structure may be a diamond nanopillar as described above.

In some embodiments, the distance between the output end of the opticaloutcoupling structure and the spin defect may be preferably less than50, 40, 30, 20, 10, or 5 nm.

In some embodiments, the distance between the output end of the opticaloutcoupling structure and the spin defect may be about 1 μm. In someembodiments, the distance between the output end of the opticaloutcoupling structure and the spin defect may be between 0.5 μm and 10μm.

In some embodiments, the optical outcoupling structure may have adiameter between 100 nm and 300 nm, and a length between 0.5 μm and 5μm.

FIGS. 12A, 12B, 12C, 12D, 12E, and 12F illustrate alternativeembodiments of nanosensors based on spin defects. FIG. 12A illustratesan optical outcoupling structure based on single crystal diamond. FIG.12B illustrates an AFM-based set up as described in earlier paragraphs,but with an NV centre formed in monolithic diamond, with no nanopillarand using total internal reflection to direct the fluorescence towardsthe detector.

The embodiment illustrated in FIG. 12C likewise does not include ananopillar. Rather, the optical outcoupling structure in FIG. 12Clikewise is based on single crystal diamond, and includes an SIL (solidimmersion lens) in the optical path of the NV centre. The SIL might beprocessed in the diamond, or be a separate diamond SIL attached or a SILfabricated in another material. FIG. 12D illustrates an alternativegeometry, again based on monolithic diamond and not including ananopillar, but using total internal reflection to direct thefluorescence towards the detector. FIG. 12E illustrates a configurationin which the optical outcoupling structure can be mounted without acantilever. In FIG. 12F, the NV centre is formed using as-grown nitrogenvacancy defects within the diamond material or nitrogen-vacancy defectsformed by conversion of as-grown nitrogen into nitrogen vacancy defectsby irradiation and annealing (rather than via ion-implanted nitrogen).

FIGS. 13A-13G illustrate back-etching that allows the NV distance to befurther reduced. As described above, one of the most importantparameters of an NV-based scanning magnetometer is the distance betweenthe NV centre and the end of the tip of the scanning probe it isattached to.

This distance is important because it sets a lower bound for theachieved spatial resolution in magnetic imaging. Further, magnetic fieldstrengths typically fall off rapidly with distance, so for a givenmagnetic sensitivity, data acquisition times fall off rapidly withdecreasing distance. When measuring magnetic dipole fields, such asthose from a single spin, the strength of the field falls with the cubeof distance. Since the signal-to-noise of measurements goes as thesquare-root of integration time, the needed data acquisition time thusscales with distance to the inverse power of six (d⁻⁶)—i.e. if theNV-to-target distance can be reduced by a factor of two, the neededintegration time decreases by a factor of 64.

In order to optimize the performance of NV magnetometers, it istherefore important to minimize the distance between the NV defect andthe end of scanning tip, for example the distal end (or output end) ofthe nanopillar. An established method for creating shallow NV centres isthrough nitrogen implantation and subsequent annealing. The stochasticnature of this method however, leads to an a priori unknown NV-sampledistance for a given device containing a single NV centre. Moreover, NVyield and quality fall off rapidly with implanted distance depth and sothe NV centres cannot be brought arbitrarily close to the samplesurface.

In some embodiments, to achieve acceptable yields and magneticsensitivities, nitrogen may be implanted with 6 keV energy at a dose of3e¹¹ N/cm². An implantation energy of 6 keV nominally forms a layerdepth of 10 nm below the surface. The experimental methods describedabove indicate, however an average depth of roughly 25 nm.

In some embodiments to reduce the NV-to-sample distance a back-etchingmethod may be used which iterates between accurately measuring theNV-to-sample distance and then carefully etching away the very end ofthe diamond probe. With this scheme this distance may be reducedsignificantly, e.g. by a factor of 2.3. importantly, the spin propertiesof the NV sensor were maintained during this reduction (T₁, T₂, NVcontrast, NV counts), so the NV sensitivity was preserved during theetch. Thus, the required integration time for single-spin imaging wasreduced by a factor of roughly 140. The achieved single spin imagingcould achieve a signal-to-noise ratio of 1 in 2 minutes of integrationtime. With this back-etching, the imaging may now be performed inroughly second of integration time. This is an important part of thisinvention such that a signal to noise ratio of two for a single spin isachieved for an experimental integration of time of preferably less than10 mins, 5 mins, 3 mins, 2 mins, 1 min, 30 seconds, 15 seconds, 10seconds, 5 seconds, 2 seconds, 1 second, 0.5 second.

To measure the distance between the NV and the end of the scanning tip,in some embodiments a method may be used that is based on measuring theextent of fluorescence quenching into a graphene monolayer. When anoptical emitter, such as an NV centre or other spin defect, is broughtinto close proximity of a metal (typically within tens of nanometers)instead of emitting into the optical far-field, some fluorescence isemitted into the metal, creating either plasmons or electron-hole pairs.

Typically, this fluorescence quenching changes rapidly with distance(proportional to d⁻⁴), and so is a very sensitive measure of distance.If a graphene layer is used as the metal, because its AC conductivity iswell known, the calibration between distance and the quenching amountcan be quantified, which allows for a precise determination of distance.In practice, this is achieved by measuring the fluorescence from the NVcentre away from a graphene flake, and then comparing how much thefluorescence decreases after it is scanned over a graphene monolayerflake. Typically the graphene flake is separated laterally, so it isstill on the substrate.

With the NV-to-sample distance determined, the end of the nanopillar tipcan be etched without fear of etching away the NV centre itself. In someembodiments, a process for doing this etch may be used that satisfies afew criteria: 1) The etch is slow enough to controllably etch in few(˜1-3) nanometer steps; 2) The etch will not adversely influence thecharge state of the NV by changing the surface termination (typicallyoxygen termination); and 3) The etching process works at a lowtemperature, because the etch is performed after measuring theNV-to-sample distance the NV sensor is mounted on a scanning probe whichuses a series of glues that cannot survive processing above ˜150 degreesCelsius.

In some embodiments, this process uses a weak oxygen plasma. FIG. 13Aillustrates reducing of the NV distance via oxygen plasma. The RF powermay be about 100 W, or in other alternatively may be in the range of50-150 W, with a low-bias power. The low-bias power may be <50 W. Insome embodiments, the low-bias power may be 0 W.

FIG. 13B illustrates etching on diamond tips. As indicated in FIG. 13B,etching 37.5 minutes yields nearly 50% quenching. The above-describedetch process may be performed for up to ˜20 minutes at a time. Longertimes may generate excess heating, which may melt the glues that areused. FIG. 13C illustrates quenching increase after more etching. Insome embodiments, etch rates of <1 nm/minute may be achieved. These etchrates may be calibrated by etching a region of diamond, partiallycovered by an etch mask, and then measuring the resulting diamondprofile with an AFM. In some embodiments, the machine used for doing theetching may be a Plasma stripper.

FIG. 13D illustrates graphene quenching vs. etch time, while FIG. I 3Eillustrates NV fluorescence v. etch time. FIG. 13F illustrates the ESRcounts for confirming NV distance. A summary of NV depth v. etch time isprovided in FIG. 13G.

In summary, methods and systems have been described relating tonanoscale scanning sensors with spin defects, such as single NV centres.These sensors achieve long spin coherence times, together with highmechanical robustness and high signal collection efficiencies

The methods and systems described above has many other potentialapplications. These applications include without limitation opticalsensors, as well as platforms for coherently coupling the scanning NVspin to other spin systems such as phosphorus in silicon, other NVcentres, or carbon-based spin qubits. Quantum information could therebybe transferred between a stationary qubit and the scanning NV centredescribed above, and from there to single photons or other qubit systemssuch as long-lived nuclear spin qubits in the diamond matrix.

The components, steps, features, objects, benefits and advantages thathave been discussed are merely illustrative. None of them nor thediscussions relating to them, are intended to limit the scope ofprotection in any way. Numerous other embodiments are also contemplatedincluding embodiments that have fewer, additional, and/or differentcomponents, steps, features, objects, benefits and advantages. Thecomponents and steps may also be arranged and ordered differently.

Nothing that has been stated or illustrated is intended to cause adedication of any component, step, feature, object, benefit, advantage,or equivalent to the public. While the specification describesparticular embodiments of the present disclosure those of ordinary skillcan devise variations of the present disclosure without departing fromthe inventive concepts disclosed in the disclosure.

While certain embodiments have been described it is to be understoodthat the concepts implicit in these embodiments may be used in otherembodiments as well. In the present disclosure, reference to an elementin the singular is not intended to mean “one and only one” unlessspecifically so stated, but rather “one or more.” All structural andfunctional equivalents to the elements of the various embodimentsdescribed throughout this disclosure, known or later come to be known tothose of ordinary skill in the art, are expressly incorporated herein byreference.

What is claimed is:
 1. An atomic force microscope tip formed of adiamond material, the atomic force microscope tip comprising: one ormore spin defects configured to emit fluorescent light; and an opticaloutcoupling structure formed by the diamond material and configured tooptically guide the fluorescent light emitted by the one or more spindefects toward an output end of the optical outcoupling structure,wherein the one or more spin defects are located no more than 50 nm froma sensing surface of the atomic force microscope tip; and wherein theatomic force microscope tip including the optical outcoupling structureis formed of a diamond component having at least one linear dimensiongreater than 1 μm in length.
 2. The atomic force microscope tip of claim1, wherein the one or more spin defects are located no more than 40 nm,30 nm, 20 nm, 15 nm, 12 nm, or 10 nm from the sensing surface of theatomic force microscope tip.
 3. The atomic force microscope tip of claim1, wherein the one or more spin defects are NV- (nitrogen-vacancy)defects.
 4. The atomic force microscope tip of claim 1, wherein adecoherence time of the one or more spin defects is greater than 10μsec, 50 μsec, 100 μsec, 200 μsec, 300 μsec, 500 μsec, or 700 μsec. 5.The atomic force microscope tip of claim 1, wherein the atomic forcemicroscope tip including the optical component is formed of a singlecrystal diamond material.
 6. The atomic force microscope tip of claim 1,wherein the optical outcoupling structure is formed of a nanopillar. 7.The atomic force microscope tip of claim 6, wherein the nanopillar has adiameter between 100 nm and 300 nm, and a length between 0.5 μm and 5μm.
 8. The atomic force microscope tip of claim 1, wherein the atomicforce microscope tip comprises no more than 50, 30, 10, 5, 3, 2, or 1spin defects located no more than 50 nm from the sensing surface andoptically coupled to the optical outcoupling structure.
 9. The atomicforce microscope tip of claim 1, wherein the atomic force microscope tipcomprises more than 50 spin defects in the form of a layer located nomore than 50 nm from the sensing surface and optically coupled to theoptical outcoupling structure.
 10. A system comprising: an atomic forcemicroscope tip formed of a diamond material, the atomic force microscopetip comprising one or more spin defects configured to emit fluorescentlight, and an optical outcoupling structure formed by the diamondmaterial and configured to optically guide the fluorescent light emittedby the one or more spin defects toward an output end of the opticaloutcoupling structure, wherein the one or more spin defects are locatedno more than 50 nm from a sensing surface of the atomic force microscopetip, and wherein the atomic force microscope tip including the opticaloutcoupling structure is formed of a diamond component having at leastone linear dimension greater than 1 μm in length; an optical excitationsource configured to generate excitation light directed to the one ormore spin defects causing the one or more spin defects to fluoresce; anoptical detector configured to detect the fluorescent light that isemitted from the one or more spin defect and that exits through theoutput end of the optical outcoupling structure after being opticallyguided therethrough; and a mounting system comprising an atomic forcemicroscope and configured to hold the atomic force microscope tip andcontrol a distance between the sensing surface of the atomic forcemicroscope tip and a surface of a sample while permitting relativemotion between the sensing surface of the atomic force microscope tipand the sample surface.
 11. The system of claim 10, comprising anoptical microscope coupled to the mounting system and configured tooptically address and readout the one or more spin defects.
 12. Thesystem of claim 10, further comprising a microwave source, and whereinthe microwave source is configured to generate microwaves tuned to aresonant frequency of at least one of the spin defects.
 13. The systemof claim 12, wherein the one or more spin defects are NV defects, andwherein the system is configured to detect an external magnetic field bymeasuring a Zeeman shift of a spin state of the NV defects.
 14. Thesystem of claim 13, wherein the microwaves comprise a spin-decouplingsequence of pulses, and wherein the sequence includes at least one of: aHahn spin-echo pulse sequence; a CPMG (Carr Purcell Meiboom Gill) pulsesequence; an XY pulse sequence; and a MREVB pulse sequence.
 15. Thesystem of claim 10, wherein the system is configured to have an ACmagnetic field detection sensitivity better than 200, 100, 75, 60, 50,25, 10, or 5 nT Hz-1/2.
 16. The system of claim 10, wherein the systemis configured to have a DC magnetic field detection sensitivity betterthan 50, 20, 10, 6, 4, 1, or 0.5 μT Hz-1/2.
 17. The system of claim 10,wherein the system is configured to resolve single spin defects in asample.
 18. The system of claim 10, wherein required integration timefor single spin imaging with a signal to noise ratio of 2 is less than 5mins, 3 mins, 2 mins, 1 min, 30 seconds, 15 seconds, 10 seconds, 5seconds, 2 seconds, 1 second, or 0.5 second.